Hooded anode x-ray tube with tilted target



Sept. 25, 1956 z. J. ATLEE 2,764,706

HOODED ANODE X-RAY TUBE WITH TILTED TARGET Filed Dec. 26, 195 2sheets-sheet 1 I2 FILAMENT TEMPERATURE INVEN TOR. ZE D J ATLEE 2 l"ATTORNEYS Z. J. ATLEE Sept; 25, 1956 HOODED ANODE X-RAY TUBE WITH TILTEDTARGET Filed Dec. 26, 1952 2 Sheets-Sheet 2 INVENTOR. ZED J. ATLEE GiddCALM ATTORNEYS Unite tates Patent 2 HOODED ANODE X-RAY TUBE WITH TILTEDTARGET Zed J. Atlee, Chicago, Ill., assignor to Dunlee Corporation,Chicago, Ill., a corporation of Illinois Application December 26, 1952,Serial No. 327,969

7 Claims. (Cl. 313-56) The present invention relates to X-ray generatorsand more particularly to improvements in X-ray generators of the hoodedanode type.

Anode hoods, that is, tubular shields, surrounding the target element ofan anode and projecting toward a cathode, are desirable in X-raygenerators to intercept all but the useful portion of the generatedX-rays, thus making it possible to reduce the amount of externalprotection required around the generator. An anode hood also shields theenvelope from bombardment by electrons originating from the anode orother stray electrons which will weaken the envelope and may evenpuncture the same. The hood also constricts the paths of the lines ofpotential between the cathode and anode making it possible to increasethe potential capacity of the generator.

In the normal operation of an X-ray generator having an elongateelectron emitting filament, only the central portion of the filament isheated to electron emissive temperature by the heating current. However,in X-ray generators as provided heretofore, the arrangement of thetarget on the anode is such that inverse electrons from the target mayimpinge upon one of the filament end portions heating that portion abovethe emission threshold temperature. It has been necessary, therefore, toprovide an opening in the anode hood sufficiently large to admit theelectrons generated in this end portion of the filament. A large openinglessens the possible shielding efliciency of the hood, of course, andalso permits some divergence of the lines of potential between the anodeand cathode so that the generator does not operate at peak efficiency.

It is an object of the present invention, therefore, to provide an X-raygenerator having a construction whereby improved hood-shieldingcharacteristics may be obtained.

More particularly, it is an object of the present invention to providean X-ray generator of the hooded anode type having a hood electronopening of relatively small cross-sectional area.

Other objects and advantages of the present invention will become moreapparent hereinafter.

In accordance with the illustrated embodiment of the present invention,the target face of the anode is provided with the conventional angle ofinclination in a direction longitudinally of the cathode filament and inaddition with a second angle of inclination in the direction at rightangles with respect to the plane of the first angle whereby the inverseelectron emission from the focal spot on the target of the anode willimpinge harm lessly against the inner surface of the anode hood, oragainst the cathode in a laterally displaced relation with respect tothe filament. Since only the central portion of the filament will beheated to an electron emissive temperature, the axial opening of theanode hood need be only of a diameter substantially equal to theemissive length of the filament, which opening may therefore be2,764,706 Patented Sept. 25, 1956 made substantially smaller than in theX-ray generators prior hereto.

For a more detailed description of the invention, reference is made tothe following specification taken in connection with the accompanyingdrawing, wherein:

Fig. 1 is a fragmentary side elevation, partly in cross section, of anX-ray generator made in accordance with the present invention;

Fig. 2 is a cross-sectional view along line 2-2 of Fig. 1;

Fig. 3 is a view showing the emission characteristics of an electrongenerating filament under various conditions;

Fig. 4 is a fragmentary side elevation, partly in cross section, of ahooded X-ray generator made in accordance with the prior art;

Fig. 5 is a fragmentary view, partly in cross section, of a modifiedform of X-ray generator embodying the present invention;

Fig. 6 is a cross-sectional view taken substantially along line 66 ofFig. 5;

Fig. 7 is an end view of the cathode taken along line 77 of Fig. 5;

Fig. 8 is an end View of the anode taken along line 8-8 of Fig. 5;

Fig. 9 is a cross-sectional view of the anode taken substantially alongline 9-9 of Fig. 6 and Fig. 10 is an end view showing a modified hoodconstruction.

Referring first to Fig. 3, a cathode filament is illustrateddiagrammatically at 10, the normal temperature characteristic of theheated filament being illustrated by the curve 11 in the adjacent graph,in which graph the ordinate represents the longitudinal extent of thefilament and the abscissa represents the relative temperature. Asindicated by the curve 11, the opposite end portions 12 of the filament10 are normally relatively coo-l in respect to the medial portion 13lying between the dotted lines 14. Normally an X-ray generator will beoperated so that only the medial portion 13 of the filament will be atan electron emitting temperature. heretofore provided, however, and. asillustrated in Fig. 4, inverse electrons emitted from the anode targetin the direction at right angles to the planar surface thereof, mayimpinge upon the lower portion of the filament to heat the same aboveits electron emissive temperature as indicated by the dotted line 15 inFig. 3. Referring to Fig. 4 which illustrates the heretoforeconventional arrangement of a cathode 16 and anode 17 having a hood 18and in which arrangement the planar anode face is inclined with respectto the cathode face only at the angle a, it will be observed that thebeam 19 of inverse electrons emitted from the focal spot on the anodetarget 20 may impinge upon the lower end 21 of the cathode filament.Such impingement causes localized heating of the filament end 21 toincrease the emission of electrons from that portion of the filament asmentioned above. cause of the emission of the electrons from the lowerend portion 21 of the filament 22, the opening 23 in thewould cause itto heat, generating copper vapor which would deposit upon the anodetarget as well as upon the interior surface of the X-ray generatorenvelope to effectively reduce the useful life thereof.

Referring now to Fig. 1', there is shown an X-ray gen- In X-ray tubesaserator in accordance with the present invention and including anenvelope 24 and an anode unit 25 and a cathode unit 26 mounted coaxiallyWithin the envelope in any suitable manner. The cathode unit 26 includesa head having a planar surface 33 facing the anode and extendingperpendicularly to the axis of the units. The cathode head is formedwith an elongated electron focusing cup recess 34 therein in which ismounted an elongated filament 35 which extends in a plane perpendicularto the axis of the unit, the filament 35 having a conventionalcharacteristic temperature curve such as the curve 11 in the graph ofFig. 3, whereby electrons will be emitted only from the medial portionof the filament and will be focused upon the target element 27 over anelongated focal area 36.

The anode unit 25 comprises a planar faced target element 27 and atubular shield or hood 28 mounted on the anode coaxial with the anodeaxis and extending toward the cathode unit 26, the hood 28 having acoaxial opening 29 in the end facing the cathode unit for the admissionof electrons. The target element 27 is inclined at the normal angle ofabout 20 degrees in respect to the longitudinal direction of thefilament 35 so that X-rays generated at the target may be transmittedtherefrom as an X-ray beam 39 through an opening 40 provided therefor inthe anode hood 28. The target element 27 is further inclined at an angleb, for example, of about 20 degrees, in respect to the plane of thecathode face in the direction at right angles to the plane of the usualangle of inclination a mentioned above as may be clearly seen in Fig. 2.As is more fully explained in my copending application Serial No.197,444, filed November 24, 1950, now Patent No. 2,671,867, and as willbe seen from Fig. 2, as a result of the second angle b of inclination,the inverse electrons generated at the target and leaving the surfacethereof at right angles and indicated by the dotted lines 41, willnormally impinge against the inner surface of the hood 28, or should anypass through opening 29, they would fall harmlessly upon the cathodesurface 33 at a point displaced laterally from the cathode filament 35.Localized heating of the filament end will not occur as in prior typesof X-ray generators and the temperature of the filament during operationof the generator will adhere to the normal characteristic curve 11 asillustrated in Fig. 3. This makes it possible to restrict the axialopening 29 in the anode hood 28 to a diameter substantially equal to oronly slightly greater than the normal effective electron emitting lengthof the filament 35 such as, for example, the length corresponding to theportion intercepted by the dotted lines 14 of Fig. 3. The emission fromeither of the opposite end portions of the filament striking the end ofthe hood 28 will not be of such magnitude as will cause the copper anodehood to heat to a temperature at which deleterious vaporization willtake place. The diameter of the axial hood opening 29 need be no greaterthan the length of the filament which has not been feasible heretoforeand, as explained above, may even be somewhat less than the filamentlength. It is preferred, however, that the hood opening be of a diametersubstantially equal to the length of the filament 35 whereby alignmentof the anode and cathode during assembly of the generator isfacilitated. The smaller opening makes the hood more effective as ashield, of course, and moreover constricts the lines of potential evenmore than with hoods of larger openings, making it possible to increasethe operating potential and thus improve the eificiency of thegenerator.

The views of Figs. to 9, inclusive, illustrate the present invention asembodied in a double focus, tilted anode type of generator such as shownand claimed in my copending application Serial No. 303,945, filed August12, 1952. Referring now more particularly to Figs. 5 and 6, there isshown a generator comprising spaced apart coaxial anode and cathodeunits 43, 44, respectively. The end of the cathode 44 facing the anodeis provided with .tion.

a pair of cathode means comprising a pair of elongated, longitudinallyparallel electron focusing cup recesses 45 and 46 and in which recessesare disposed the electron emitting filaments 47, 48, respectively. Thefilaments 47, 48 are connected by suitable means (not shown) to a sourceof electrical current whereby they may be individually energized. Thehead end of the cathode 44 is formed with two plane surfaces 49, 50disposed at a large, obtuse angle with respect to one another andintersecting along a line 51 perpendicular to the longitudinal axis ofthe cathode head but offset slightly to one side thereof. The cathodefaces 49, 59 and the recesses therein are arranged so that the electronsemitted by the cathode filaments are focused on the anode target inelongated, longitudinally parallel focal spots 54, 55 which arelaterally spaced from each other substantially diametrically on oppositesides of the anode axis but are relatively closely adjacent.

The anode unit 43 is provided with a coaxial tubular hood 41 whichextends toward the cathode unit 44. A coaxial, circular electronadmitting opening 42 is provided in the end of the hood 41 facing thecathode unit 44. Embedded in the target face of the anode is a unitaryV-shaped target means or element 56, which element is preferably made oftungsten or other X-ray emissive heavy metal having a relatively highmelting point. The target element 56 comprises a pair of planar X-rayemitting surface defining portions 5'7, 58 defining a separate targetarea for each of the electron beams generated by the filaments 47, 48.

As explained in my copending application Serial No. 303,945, the facesof the target portions 57, 58 are inclined at a small, acute angle atoward each other which angle may be, for example, about 20 degrees, andintersect along a line 59 which extends in the same direction as thefilaments but is inclined at an angle b with respect to a plane normalto the axis of the tube. The angle b may also be of the order of 20degrees. While the angle of inclination a of the anode faces 57, 58 maybe varied, it is important that each face be inclined at such an anglein respect to the electron beam focused thereon that any perpendicularfrom the focal spot on the anode target will intersect the cathode unitat a point displaced laterally from the filament focused upon theparticular target por- For example, in Fig. 5, the cathode filament 48is shown focused upon the target face portion 58 but all theperpendiculars from the focal spot 55, representing the paths of flowfor the inverse electrons emitted from the focal spot and indicated bythe dotted lines 64, strike the inner surface of the hood or should theypass through fall harmlessly upon the cathode head at a point laterallydisplaced from the filament 48.

As may be seen in Fig. 7, the filament 48 is of greater length than thefilament 47. Since localized heating of either end of the filament 48will not occur as in prior types of X-ray generators and the temperatureof the filament 48 during operation of the generator will adhere to thenormal characteristic curve 11 as illustrated in Fig. 3, it is possibleto restrict the axial electron admitting opening 42 in the anode hood 41to a diameter substantially equal to or only slightly greater than thenormal electron emitting length of the filament 48. To facilitatealignment of the anode and cathode in the assembly of the generator, theopening 42 may be of a diameter substantially equal to the length of thelonger filament 48 as shown in Fig. 6, but which diameter issubstantially lesser than the diameter of the openings providedheretofore. As in the case of the previous modification, the smallopening makes the hood more eifective as a shield and constricts thelines of potential to an even greater degree than is experienced withhoods of larger openings, making it possible to increase the operatingpotential and efficiency of the generator.

The hood member 41 is also provided with an elliptical opening 68 in theside thereof to provide, a. path for the X-ray beams 62, 63. Theelliptical opening 68 is positioned with the major axis thereof,indicated in Fig. by dotted line 69 extending transversely of the anodeand lying in a plane normal to the anode axis and substantiallybisecting the focal spots 54, 55. Thus an opening is provided with itsgreater width in the direction necessary to permit passage of a usefulportion of the X-rays generated on the target element but with its widthrestricted in the opposite direction to effect maximum shielding of theexternal X-ray generator structure from stray electrons and otherportions of the generated X-rays.

In Fig. is shown a modified hood construction in which the electronadmitting opening 70 is rectangular. This construction may in someinstances be preferred to more positively preclude the possibility ofintercepting electrons from the cathode filaments. The rectangularopening 70 provides greater area laterally of the center portion of abeam, and especially at the opposite ends of the beam.

Having illustrated and described a preferred embodiment of theinvention, it should be apparent to those skilled in the art that theinvention permits of modification in arrangement and detail. I claim asmy invention all such modifications as come within the true spirit andscope of the appended claims.

I claim:

1. In an X-ray generator an anode unit having a target, a cathode, and atubular shield surrounding said target and extending in the direction ofsaid cathode, said cathode comprising an elongated electron emittingfilament focused to project electrons upon a planar faced focal spotarea on said target in a beam substantially coaxial with said shield,said focal spot area being angularly inclined with respect to thedirection of said beam whereby the perpendicular from any point on saidfocal spot area intersects said cathode at a point displaced laterallyfrom said filament, said shield having an electron admission openingthrough the end thereof of a diameter no greater than the length of saidfilament.

2. An X-ray tube comprising coaxially mounted anode and cathode units,said anode unit comprising a planar faced target element and a tubularhood member coaxial with the anode axis and extending toward saidcathode unit, said cathode unit including an electron generating meansfocused upon said target element and comprising an elongate filamentextending at right angles to the axis of said cathode unit, said targetelement being inclined in the direction at right angles with respect tothe longitudinal direction of said filament whereby the perpendicularprojection of the focal spot on said target element is wholly displacedlaterally of said electron generating means, said hood member having anelectron admission opening in the end thereof of a diameter less thanthe length of said filament.

3. An X-ray generator comprising coaxially mounted anode and cathodeunits facing each other in spaced apart relation, said cathode unitincluding a head having an elongated electron generating filamentmounted thereon, the longitudinal axis of said filament being at rightangles to the axis of said cathode unit, said cathode unit axisintersecting said filament axis substantially medially of the filament,said anode unit having a substantially planar faced target elementmounted thereon, the plane of said target element being inclined at anacute angle in a plane at right angles with respect to the longitudinaldirection of said filament whereby the perpendicular from the focal spotof the target element intersects said cathode element at a pointdisplaced laterally from said filament, said anode unit including atubular hood surrounding said target element, and projecting toward saidcathode, said hood having a circular, axial opening in the end thereofadjacent said cathode unit having a diameter only slightly greater thanthe normal effective electron emitting length said filament.

4. An X-ray tube comprising coaxially mounted anode and cathode units,said anode unit comprising a planar faced target element and a tubularhood coaxial with the anode axis and extending toward said cathode unit,said cathode unit comprising a head having a planar surface facing saidanode and extending perpendicularly to the axis of said units, said headhaving an elongated electron focusing cup recess formed therein, anelongated filament mounted in said recess and extending in a planeperpendicular to said axis whereby electrons emitted from said filamentwill be focused upon said target element over an elongated focal area,the plane of said target element being inclined at a first angle ofapproximately 20 degrees in respect to said cathode surface and to thelongitudinal direction of said filament, and at a second angle of 20degress in respect to said cathode surface in a direction at rightangles in respect to the plane of said first angle, said hood having acircular, axial opening in the end thereof not substantially greaterthan the normal effective electron emitting length of said filament.

5. An X-ray generator comprising spaced apart coaxial anode and cathodeunits, said cathode unit including a head having a pair of laterallyspaced apart electron generating means therein each comprising anelongated electron focusing cup recess formed in said head and anelongate filament mounted in the recess, said electron generating meansbeing focused to project their respective electron beams upon elongate,laterally spaced apart anode focal spots, said anode unit includingtarget means defining a separate planar target area for each of saidfocal spots, the face plane of each of said target areas being inclinedat such angle to the respective beam that all perpendiculars from thefocal spot of a target area are displaced laterally from the electrongenerating means focused thereon, and an anode hood member projectingtoward said cathode unit, said hood member having an electron admittingopening having a greatest dimension in the direction parallel to saidfilaments no greater than the length of the longer of said filaments.

6. An X-ray generator comprising spaced apart coaxial anode and cathodeunits, said cathode unit including a head having a pair of laterallyspaced apart electron generating means therein each comprising anelongate electron focusing cup recess formed in said head and anelongate filament mounted in the recess, said electron generating meansbeing focused to project their respective electron beams upon elongate,laterally spaced apart anode focal spots, said filaments being ofdifferent lengths, said anode unit including target means defining aseparate planar target area for each of said focal spots, the face planeof each of said target areas being inclined at such angle to therespective beam that all perpendiculars from the focal spot of a targetarea are displaced laterally from the electron generating means focusedthereon, said anode unit including an anode hood member coaxial with theanode axis and projecting toward said cathode unit, said hood memberhaving a coaxial, circular electron admitting opening having a diameternot substantially greater than the normal effective length of the longerof said filaments.

7. In an X-ray generator comprising spaced apart coaxial anode andcathode units, said cathode units including a pair of elongatesubstantially parallel cathode filaments of unequal lengths focused onspaced apart anode focal spots spaced substantially diametrically onopposite sides of the anode axis, said anode unit having a pair ofplanar X-ray emitting surface defining portions obliquely inclined inrespect to each other and to the direction of said filaments whereby theaxes of the paths of the X-ray beams from said planar portions aresubstantially parallel to one another and substantially at right anglesto the anode axis, said anode unit including a tubular hood memberprojecting toward said cathode unit, said hood member having a circularelectron emitting opening having a diameter not substantially greaterthan 7 the length of the longer of said filaments, said hood memberhaving an elliptical opening in the side thereof in the path of saidX-ray beams, said elliptical Opening being positioned with its majoraxis lying in a plane normal to the anode axis and substantiallybisecting said focal spots. 5

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